Uranium has multiple isotopes, all of which are radioactive. Naturally occurring uranium nominally contains 99.28305% U-238, 0.7110% U-235, and 0.0054% U-234 by weight. The concentration of fissile U235 nuclei in natural uranium is too low for the use of natural uranium as fuel in a light water reactor (LWR). Thus, isotope enrichment is necessary to increase the concentration of fissile U235 nuclei to a suitable level. In particular, for use in a LWR, the percentage of the fissile uranium isotope U235 has to be raised from the natural state of 0.71% to a reactor grade concentration of approximately 3%-5%.
Commercially available enrichment technologies include the gaseous diffusion process and the centrifuge process. Both of these processes require the prior conversion of uranium to the gaseous form of uranium hexafluoride (UF6). Uranium ore is first mined and produced into uranium oxide (often called “yellowcake”). The uranium oxide is then combined with anhydrous hydrogen fluoride (“HF”) and fluorine gas in a series of chemical reactions to form the chemical compound UF6. Fluorine is a pale yellow, corrosive gas, which reacts with most organic and inorganic substances; elemental fluorine and the fluoride ion are highly toxic. The UF6 is then enriched to increase the concentration of fissile U235 nuclei.
Uranium hexafluoride is used in uranium processing because its ability to be a solid, liquid, or gas, depending on temperatures and pressures commonly used in industrial processes make its use very convenient. The product stream of enriched UF6 obtained is converted to UO2 for further processing to nuclear fuel assemblies.
The enrichment process not only produces the enriched product, but also generates a waste stream or tails of uranium hexafluoride depleted in U235, typically with a concentration of 0.2%-0.35%. It is important to note that these tails represent a significant percentage of the fissile uranium isotope U235 found in naturally occurring uranium which is not extracted by conventional enrichment processes.
ASTM C787-06 “Standard Specification for Uranium Hexafluoride for Enrichment,” ASTM International, deals with uranium hexafluoride (UF6) intended for feeding to an enrichment plant and is incorporated by reference as if fully set out herein. Described therein is UF6 derived from unirradiated natural uranium and UF6 derived from irradiated uranium that has been reprocessed and converted to UF6 for enrichment and subsequent reuse. The reference provides information: to define the impurity and uranium isotope limits for commercial UF6 feedstock so that the corresponding enriched uranium is essentially equivalent to enriched uranium made entirely from virgin natural UF6; and to define additional limits for reprocessed UF6 (or any mixture of reprocessed UF6 and commercial UF6).
Present enrichment processes utilize large volumes of low density feedstock which is enriched within large expansive devices. These processes result in limited efficiency and/or enrichment, as well as the creation of massive contaminated structures which require extremely careful handling and long term disposal for safety reasons.
Therefore there is a need for an enrichment process capable of handling higher density feed stock as well as reducing the number/size of the devices and the contaminated structures which require extremely careful handling and long term disposal. There is also a need for an enrichment process that is more efficient than conventional enrichment processes.